Fisheye lens

ABSTRACT

An object of the present invention is to provide a fisheye lens, in spite of its compactness, having an angle of view of 180 degrees and a fast f-number of 2.8 with securing sufficient back focal length and suitable for a digital camera. The fisheye lens consists of a front lens group G 1  having negative refractive power and a rear lens group G 2  having positive refractive power locating with a space along the optical axis apart from the front lens group G 1 . The front lens group G 1  includes a plurality of negative lens components, L 1  and L 2 . The rear lens group G 2  includes at least one cemented lens L 5 , and given conditional expressions are satisfied.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/631,760filed Aug. 1, 2003 now U.S. Pat. No. 6,844,991.

This application claims the benefit of Japanese Patent applications No.2002-224994, No. 2002-225001, No. 2003-051432 and No. 2003-197315 whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fisheye lens sufficiently securing anequivalent air distance between the last lens surface and the imageplane, in spite of its compactness, suitable for a SLR camera and inparticular for a SLR digital camera.

2. Related Background Art

Almost all fisheye lenses for a SLR camera have an image sizecorresponding to 35-mm films (For example, Japanese Patent ApplicationLaid-Open No. 54-32319.) Even if these fisheye lenses are directly usedin a SLR digital camera, the angle of view of 180 degrees cannot besecured because due to the image size the relation between the focallength and the angle of view of a SLR digital camera is different fromthat of a SLR film camera.

Generally, the size of the imaging plane of an imaging device for a SLRdigital camera is a little smaller than the film frame size of 35-mmfilms. Accordingly, “the diagonal of the image size of 35-mm film frame”divided by “the diagonal of the image size of the imaging device”multiplied by the focal length of the lens becomes the focal length(converted focal length to 35-mm film format) of the lens used in a SLRdigital camera.

Therefore, when a fisheye lens for a SLR film camera having the angle ofview of 180 degrees is directly used in a SLR digital camera, no morethan n effect of a super wide-angle lens can be obtained.

As described above, the angle of view of a SLR digital camera whoseimage size is a little smaller than that of a SLR film camera inevitablybecomes narrow. Moreover, when you want to take a photograph of a widearea with use of a fisheye lens, the focal length has to be set evenshorter. As a result, since the back focal length of a fisheye lens fora SLR digital camera is required to become more than 3 times as large asthe focal length, an extremely strong divergent lens group has to belocated to the object side of the lens system.

In a fisheye lens, a retrofocus type in which the principal point islocated backward is used. However, since an extremely strong divergentlens group is located to the object side of the lens system, opticalperformance tends to be deteriorated due to curvature of field andastigmatism.

Moreover, in the retrofocus type, the diameter of the negative lens inthe front lens group tends to become large, so that it is a problem thatthe fisheye lens becomes larger and heavier.

SUMMARY OF THE INVENTION

The present invention is made in view of the aforementioned problems andhas an object to provide a fisheye lens, in spite of its compactness,having superb optical performance with well corrected curvature of fieldsufficiently secured the back focal length suitable for a SLR digitalcamera.

According to one aspect of the present invention, a fisheye lensconsists of a front lens group having negative refractive power and arear lens group having positive refractive power locating with a spacealong the optical axis apart from the front lens group. The front lensgroup includes, in order from an object, a plurality of negative lenscomponents. The rear lens group includes at least one cemented lens, andthe following conditional expression is satisfied:4.0≦Σd/f≦10.0  (1)where f denotes the focal length of the fisheye lens and Σd denotes thedistance from the most object side lens surface to the most image sidelens surface of the fisheye lens when focusing at infinity.

In one preferred embodiment of the present invention, the front lensgroup includes at least one cemented lens.

In one preferred embodiment of the present invention, the followingconditional expression is preferably satisfied;1.5≦f2/f≦4.0  (2)where f2 denotes the focal length of the rear lens group.

In one preferred embodiment of the present invention, the rear lensgroup includes, in order from the object, a positive lens component, acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a positive lens component. Thefollowing conditional expression is preferably satisfied;0.5<d1/f<2.0  (3)where d1 denotes the distance along the optical axis between the frontlens group and the rear lens group.

In one preferred embodiment of the present invention, elements in therear lens group the following conditional expression is preferablysatisfied;30≦vRP−vRN≦60  (4)where vRP denotes the mean value of Abbe numbers of the positive lenselements in the lens components in the rear lens group and vRN denotesthe mean value of Abbe numbers of the negative lens elements in the lenscomponents in the rear lens group.

In one preferred embodiment of the present invention, the followingconditional expression is preferably satisfied;0.2≦nRN−nRP≦0.45  (5)where nRN denotes the mean value of refractive indices of the negativelens elements in the lens components in the rear lens group at d-line(λ=587.6 nm) and nRP denotes the mean value of refractive indices of thepositive lens elements in the lens components in the rear lens group atd-line (λ=587.6 nm).

According to another aspect of the present invention, a fisheye lensconsists of a front lens group having negative refractive power and arear lens group having positive refractive power locating with a spacealong the optical axis apart from the front lens group. The front lensgroup includes, in order from an object, a plurality of negative lenscomponents, and two sets of cemented lens components. At least one ofthe two sets of cemented lens components is arranged a negative lenselement to the object side and the negative lens element satisfies thefollowing conditional expression:1.0<|R|/f<4.0  (6)where f denotes the focal length of the fisheye lens and R denotes theradius of curvature of the object side surface of the negative lenselement arranged to the object side.

In one preferred embodiment of the present invention, the front lensgroup consists of, in order from the object, a first lens componenthaving a negative meniscus shape with a convex surface facing to theobject, a second lens component having a negative meniscus shape with aconvex surface facing to the object, a third lens component being acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a fourth lens component being acemented lens component constructed by a positive lens element cementedwith a negative lens element. The plurality of negative lens componentsare composed of the first lens component and the second lens component,and the two cemented lens components are composed of the third lenscomponent and the fourth lens component.

In one preferred embodiment of the present invention, the front lensgroup includes, in order from the object, a first lens component havinga negative meniscus shape with a convex surface facing to the object, asecond lens component having a negative meniscus shape with a convexsurface facing to the object, a third lens component being a cementedlens component constructed by a negative lens element cemented with apositive lens element, a fourth lens component being a positive lenselement, and a fifth lens component being a cemented lens componentconstructed by a positive lens element cemented with a negative lenselement. The plurality of negative lens components are composed of thefirst lens component and the second lens component, and the two sets ofcemented lens components are composed of the third lens component andthe fifth lens component.

In one preferred embodiment of the present invention, the front lensgroup includes, in order from the object, a first lens component havinga negative meniscus shape with a convex surface facing to the object, asecond lens component having a negative meniscus shape with a convexsurface facing to the object, a third lens component being a positivelens element, a fourth lens component being a cemented lens componentconstructed by a negative lens element cemented with a positive lenselement, and a fifth lens component being a cemented lens componentconstructed by a positive lens element cemented with a negative lenselement. The plurality of negative lens components are composed of thefirst lens component and the second lens component, and the two sets ofcemented lenses are composed of the fourth lens component and the fifthlens component.

In one preferred embodiment of the present invention, the followingconditional expressions are preferably satisfied;4.0≦Σd/f≦10.0  (7)1.5≦f2/f≦4.0  (8)where Σd denotes the distance from the most object side lens surface tothe most image side lens surface of the fisheye lens when focusing atinfinity and f2 denotes the focal length of the rear lens group.

In one preferred embodiment of the present invention, the rear lensgroup includes, in order from the object, a positive lens component, acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a positive lens component. Thefollowing conditional expression is preferably satisfied;0.5<d1/f<2.0  (9)where d1 denotes the distance along the optical axis between the frontlens group and the rear lens group.

In one preferred embodiment of the present invention, the cemented lensin the rear lens group is preferably satisfied either one or both of thefollowing conditional expressions;30≦vRP−vRN≦60  (10)0.2≦nRN−nRP≦0.45  (11)where vRP denotes the mean value of Abbe numbers of the positive lenselements in the lens components in the rear lens group, vRN denotes themean value of Abbe numbers of the negative lens elements in the lenscomponents in the rear lens group, nRN denotes the mean value ofrefractive indices of the negative lens elements in the lens componentsin the rear lens group at d-line (λ=587.6 nm) and nRP denotes the meanvalue of refractive indices of the positive lens elements in the lenscomponents in the rear lens group at d-line (λ=587.6 nm).

According to another aspect of the present invention, a fisheye lensconsists of a front lens group having negative refractive power and arear lens group having positive refractive power locating with a spacealong the optical axis apart from the front lens group. The front lensgroup includes, in order from an object, a plurality of negative lenscomponents and a cemented lens component. The rear lens group includes acemented lens component and the following conditional expression issatisfied:Bf/f≧3.45  (12)0.5<d1/f<2.0  (15)where Bf denotes the back focal length of the fisheye lens, d1 denotesthe distance along the optical axis between the most image side lenssurface of the front lens group and the most object side lens surface ofthe rear lens group, and f denotes the focal length of the fisheye lens.

In one preferred embodiment of the present invention, the followingconditional expression is preferably satisfied;4.0≦Σd/f≦10.0  (13)where f denotes the focal length of the fisheye lens and Σd denotes thedistance from the most object side lens surface to the most image sidelens surface of the fisheye lens when focusing at infinity.

In one preferred embodiment of the present invention, the front lensgroup includes, in order from the object, a first lens component havinga negative meniscus shape with a convex surface facing to the object, asecond lens component having a negative meniscus shape with a convexsurface facing to the object, and a third lens component being acemented lens component constructed by a negative lens element cementedwith a positive lens element.

In one preferred embodiment of the present invention, the front lensgroup includes, in order from the object, a first lens component havinga negative meniscus shape with a convex surface facing to the object, asecond lens component having a negative meniscus shape with a convexsurface facing to the object, a third lens component being an onlynegative lens element, and a fourth lens component being a cemented lenscomponent constructed by a positive lens element cemented with anegative lens element.

In one preferred embodiment of the present invention, the front lensgroup includes, in order from the object, a first lens component havinga negative meniscus shape with a convex surface facing to the object, asecond lens component having a negative meniscus shape with a convexsurface facing to the object, a third lens component being an onlypositive lens element, and a fourth lens component being a cemented lenscomponent constructed by a positive lens element cemented with anegative lens element.

In one preferred embodiment of the present invention, the rear lensgroup includes, in order from the object, a positive lens component, acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a positive lens component. Thefollowing conditional expressions are preferably satisfied;1.5≦f2/f≦4.0  (14)where f2 denotes the focal length of the rear lens group.

In one preferred embodiment of the present invention, the cemented lensin the rear lens group is preferably satisfied the following conditionalexpressions;30≦vRP−vRN≦60  (16)0.2≦nRN−nRP≦0.45  (17)where vRP denotes the mean value of Abbe numbers of the positive lenselements in the lens components in the rear lens group, vRN denotes themean value of Abbe numbers of the negative lens elements in the lenscomponents in the rear lens group, nRN denotes the mean value ofrefractive indices of the negative lens elements in the lens componentsin the rear lens group at d-line (λ=587.6 nm) and nRP denotes the meanvalue of refractive indices of the positive lens elements in the lenscomponents in the rear lens group at d-line (λ=587.6 nm).

In one preferred embodiment of the present invention, each of all lenssurfaces of the front lens group and the rear lens group is constructedby any one of a spherical surface and a plane surface.

In one preferred embodiment of the present invention, when the focusingof the lens is moved from infinity to close object, the lens is moved tothe object side with increasing the distance between the front lensgroup and the rear lens group.

Other feature and advantages according to the present invention will bereadily understood from the detailed description of the preferredembodiments in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the lens arrangement of a fisheyelens according to Example 1 of a first embodiment and Example 15 of athird embodiment of the present invention.

FIG. 2 graphically shows various aberrations of the fisheye lensaccording to Example 1 and Example 15 of the present invention.

FIG. 3 is a sectional view showing the lens arrangement of a fisheyelens according to Example 2 of the first embodiment and Example 16 ofthe third embodiment of the present invention.

FIG. 4 graphically shows various aberrations of the fisheye lensaccording to Example 2 and Example 16 of the present invention.

FIG. 5 is a sectional view showing the lens arrangement of a fisheyelens according to Example 3 of the first embodiment and Example 17 ofthe third embodiment of the present invention.

FIG. 6 graphically shows various aberrations of the fisheye lensaccording to Example 3 and Example 17 of the present invention.

FIG. 7 is a sectional view showing the lens arrangement of a fisheyelens according to Example 4 of the first embodiment and Example 18 ofthe third embodiment of the present invention.

FIG. 8 graphically shows various aberrations of the fisheye lensaccording to Example 4 and Example 18 of the present invention.

FIG. 9 is a sectional view showing the lens arrangement of a fisheyelens according to Example 5 of the first embodiment and Example 11 of asecond embodiment of the present invention.

FIG. 10 graphically shows various aberrations of the fisheye lensaccording to Example 5 and Example 11 of the present invention.

FIG. 11 is a sectional view showing the lens arrangement of a fisheyelens according to Example 6 of the first embodiment and Example 12 ofthe second embodiment of the present invention.

FIG. 12 graphically shows various aberrations of the fisheye lensaccording to Example 6 and Example 12 of the present invention.

FIG. 13 is a sectional view showing the lens arrangement of a fisheyelens according to Example 7 of the first embodiment and Example 13 ofthe second embodiment of the present invention.

FIG. 14 graphically shows various aberrations of the fisheye lensaccording to Example 7 and Example 13 of the present invention.

FIG. 15 is a sectional view showing the lens arrangement of a fisheyelens according to Example 8 of the first embodiment and Example 14 ofthe second embodiment of the present invention.

FIG. 16 graphically shows various aberrations of the fisheye lensaccording to Example 8 and Example 14 of the present invention.

FIG. 17 is a sectional view showing the lens arrangement of a fisheyelens according to Example 9 of the first embodiment of the presentinvention.

FIG. 18 graphically shows various aberrations of the fisheye lensaccording to Example 9 of the present invention.

FIG. 19 is a sectional view showing the lens arrangement of a fisheyelens according to Example 10 of the first embodiment of the presentinvention.

FIG. 20 graphically shows various aberrations of the fisheye lensaccording to Example 10 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

A fisheye lens according to a first embodiment of the present inventionis explained below.

A fisheye lens according to the first embodiment of the presentinvention consists of a front lens group having negative refractivepower and a rear lens group having positive refractive power locatingwith a space along the optical axis apart from the front lens group. Thefront lens group includes a plurality of negative lens components. Therear lens group includes at least one cemented lens component.

The front lens group has a negative refractive power component havingstrong divergent effect to deflect light ray entering from a wideincident range of 180 degrees to a direction parallel to the opticalaxis. By constructing the negative refractive power component of thefront lens group with a plurality of negative lens components,aberrations produced thereby can be spread to or shared by the pluralityof the negative lens components.

In the first embodiment, the following conditional expression issatisfied:4.0≦Σd/f≦10.0  (1)

Furthermore, at least one of the following conditional expressions ispreferably satisfied.1.5≦f2/f≦4.0  (2)0.5<d1/f<2.0  (3)30≦vRP−vRN≦60  (4)0.2≦nRN−nRP≦0.45  (5)where f denotes the focal length of the fisheye lens, Σd denotes thedistance from the most object side lens surface to the most image sidelens surface of the fisheye lens when focusing at infinity, f2 denotesthe focal length of the rear lens group, d1 denotes the distance alongthe optical axis between the front lens group and the rear lens group,vRP denotes the mean value of Abbe numbers of the positive lens elementsin the lens components in the rear lens group, vRN denotes the meanvalue of Abbe numbers of the negative lens elements in the lenscomponents in the rear lens group, nRN denotes the mean value ofrefractive indices of the negative lens elements in the lens componentsin the rear lens group at d-line (λ=587.6 nm) and nRP denotes the meanvalue of refractive indices of the positive lens elements in the lenscomponents in the rear lens group at d-line (λ=587.6 nm).

In a fisheye lens according to the first embodiment of the presentinvention, conditional expression (1) is for controlling or determiningthe whole size and weight of the lens with securing sufficient backfocal length and correcting off-axis aberrations. When the ratio Σd/fexceeds the upper limit of conditional expression (1), the back focallength becomes too short to be able to be used for a SLR camera.Moreover, in order to pass light ray entering from a wide incident rangeof 180 degrees, the diameter (front lens diameter) of the lens locatingto the most object side becomes extremely large, so that the whole lensbecomes large in size and heavy in weight. In order to obtain goodoptical performance, it is preferable that the upper limit is set to8.0. On the other hand, when the ratio Σd/f falls below the lower limitof conditional expression (1), although the back focal length can besecured sufficiently, it becomes difficult to maintain the angle of viewof 180 degrees, so that it is undesirable. In order to obtain goodoptical performance, it is preferable that the lower limit is set to5.0.

In a fisheye lens according to the first embodiment of the presentinvention, conditional expression (2) defines an appropriate range ofthe focal length of the rear lens group with securing sufficient backfocal length and correcting off-axis aberrations. When the ratio f2/fexceeds the upper limit of conditional expression (2), it becomesdifficult to secure the angle of view of 180 degrees as well as tocorrect astigmatism and coma, so that it is undesirable. On the otherhand, when the ratio f2/f falls below the lower limit of conditionalexpression (2), the back focal length becomes too short to be able to beused for a SLR camera.

In a fisheye lens according to the first embodiment of the presentinvention, conditional expression (3) defines an appropriate range ofthe distance along the optical axis between the front lens group and therear lens group. When the ratio d1/f is higher than or equal to theupper limit of conditional expression (3), lateral chromatic aberrationbecomes too large to correct properly. In addition, the back focallength becomes too short to be able to be used for a SLR camera. On theother hand, when the ratio d1/f is less than or equal to the lower limitof conditional expression (3), it becomes difficult to secure the angleof view of 180 degrees as well as to correct astigmatism and coma, sothat it is undesirable. These results are further improved, when thelower limit of conditional expression (3) is increased to 0.7.

In a fisheye lens according to the first embodiment of the presentinvention, conditional expression (4) defines an appropriate range ofdifference in Abbe numbers between the positive lens elements andnegative lens elements of the rear lens group in order to correctlateral chromatic aberration and axial chromatic aberration produced inthe front lens group. When the value vRP−vRN exceeds the upper limit ofconditional expression (4), it becomes difficult to construct the lenssystem by available glass materials as well as to correct axialchromatic aberration. On the other hand, when the value vRP−vRN fallsbelow the lower limit of conditional expression (4), lateral chromaticaberration produced by negative lenses in the front lens group becomesdifficult to be corrected by the rear lens group and lateral chromaticaberration at g-line (λ=435.8 nm) tends to become negative, so that itis undesirable.

In a fisheye lens according to the first embodiment of the presentinvention, conditional expression (5) is for correcting curvature offield and astigmatism. By setting refractive index of the negative lensas high as possible and that of the positive lens as low as possible,Petzval sum of the whole lens system can be lowered and curvature offield and astigmatism can be suppressed to be small. When the valuenRN−nRP exceeds the upper limit of conditional expression (5), thedifference in Abbe numbers between negative lens elements and positivelens elements has to be set larger, so that it becomes difficult tocorrect axial chromatic aberration produced in the front lens group. Onthe other hand, when the value nRN−nRP falls below the lower limit ofconditional expression (5), Petzval sum of the whole lens system becomeslarge to produce large amount of curvature of field, so that it isundesirable.

By the way, by including a cemented lens component in the front lensgroup, production of lateral chromatic aberration can be suppressed.

When the focusing of the lens is carried out from infinity to closeobject, in order to prevent degradation of optical performance whenfocusing to a close object, it is preferable that the lens is moved tothe object side with increasing the distance between the front lensgroup and the rear lens group.

In the above-described construction, it is preferable that each lenssurface of the front and rear lens groups is constructed by a sphericalsurface or a plane surface without using an aspherical surface.Accordingly, it becomes easier to manufacture, assemble, and adjust eachlens element, so that manufacturing cost can be lowered.

Each example of the fisheye lens according to the first embodiment ofthe present invention is shown below.

FIGS. 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19 are sectional views showingfisheye lenses according to Examples 1 through 10 of the firstembodiment of the present invention, respectively. In respectivesectional views, a plane parallel plate P, S and I denote a filter, anaperture stop, and an image plane, respectively. The filter and theaperture stop are arranged between the front lens group G1 and the rearlens group G2. The filter that may be inserted into any place in thefisheye lens has substantially no effect on the lens. Moreover, if thefisheye lens does not have the filter, the basic optical performance ofthe lens is not substantially affected.

FIGS. 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 graphically show variousaberrations of the fisheye lenses according to Examples 1 through 10 ofthe first embodiment of the present invention, respectively. Inrespective graphs, d denotes aberration at d-line (λ=587.6 nm), gdenotes aberration at g-line (λ=435.8 nm), m denotes meridional imageplane, and s denotes sagittal image plane. FNO denotes f-number, and 2ωdenotes the angle of view (unit: degree). Distortion is defined as adeviation from an equi-solid angle projection (y=fsin(ω/2)). As is shownfrom respective graphs, each aberration is corrected preferably.

In Tables showing respective Examples, f denotes the focal length (unit:mm) of the whole lens system, FNO denotes f-number, 2ω denotes the angleof view (unit: degree), Bf denotes the back focal length (unit: mm), andTL denotes the total lens length (unit: mm). In lens data, the numberlocating at the most left side column is the surface number, r denotesradius of curvature (unit: mm), nd denotes refractive index at d-line(λ=587.6 nm), v d denotes Abbe number, and S denotes an aperture stop.

In the tables for various values, “mm” is generally used for the unit oflength such as the focal length, the radius of curvature, and theseparation between optical surfaces. However, since an optical systemproportionally enlarged or reduced its dimension can be obtained similaroptical performance, the unit is not necessary to be limited to “mm” andany other suitable unit can be used. The explanation of referencesymbols is the same in the other example.

EXAMPLE 1

A fisheye lens according to Example 1 of the first embodiment shown inFIG. 1 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, and athird lens component L3 being a cemented positive lens componentconstructed by a negative lens element cemented with a positive lenselement. The rear lens group G2 is composed of, in order from theobject, a fourth lens component L4 having a double convex shape with astronger convex surface facing to an image, a fifth lens component L5being a cemented positive lens component constructed by a negative lenselement cemented with a positive lens element, and a sixth lenscomponent L6 having a double convex shape with positive refractivepower.

Various values associating Example 1 are shown in Table 1.

TABLE 1 (Specification) f = 10.0 FNO = 2.8 2ω = 180.3 Bf = 38.0 TL =101.9 (Lens Data) Surface Number r d νd nd  1 70.000 1.50 60.68 1.60311 2 14.900 13.60 1.00000  3 36.500 1.00 49.61 1.77250  4 15.300 16.901.00000  5 −956.300 1.00 52.32 1.75500  6 10.250 3.60 31.07 1.68893  7−66.600 0.85 1.00000  8 S 8.20 1.00000  9 ∞ 1.00 64.14 1.51633 10 ∞ 3.981.00000 11 78.500 2.43 90.3 1.45600 12 −32.600 1.00 1.00000 13 −232.9001.00 23.78 1.84666 14 27.200 4.00 70.24 1.48749 15 −29.900 0.10 1.0000016 41.200 3.78 70.24 1.48749 17 −45.873 Bf 1.00000 (Values for theconditional expressions) (1) Σd/f = 6.394 (2) f2/f = 2.532 (3) d1/f =1.403 (4) νRP − νRN = 53.15 (5) nRN − nRP = 0.370

EXAMPLE 2

A fisheye lens according to Example 2 of the first embodiment shown inFIG. 3 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a double concave shape, and a fourth lenscomponent L4 being a cemented positive lens component constructed by apositive lens element cemented with a negative lens element. The rearlens group G2 is composed of, in order from the object, a fifth lenscomponent L5 having a positive meniscus shape with a stronger convexsurface facing to an image, a sixth lens component L6 being a cementednegative lens component constructed by a negative lens element cementedwith a positive lens element, and a seventh lens component L7 having adouble convex shape with positive refractive power.

Various values associating Example 2 are shown in Table 2.

TABLE 2 (Specification) f = 10.5 FNO = 2.8 2ω = 180.6 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 85.122 2.50 55.52 1.69680 2 16.625 9.29 1.00000  3 208.221 1.00 60.29 1.62041  4 28.271 17.171.00000  5 −284.146 1.00 60.09 1.64000  6 14.288 1.50 1.00000  7 18.6903.93 35.3 1.59270  8 −12.457 1.00 39.59 1.80440  9 −26.857 6.33 1.0000010 S 4.69 1.00000 11 ∞ 1.00 64.14 1.51633 12 ∞ 2.47 1.00000 13 −196.1932.15 48.87 1.53172 14 −27.112 1.00 1.00000 15 −179.223 1.00 25.431.80518 16 24.353 4.07 81.61 1.49700 17 −32.401 0.10 1.00000 18 40.9203.82 70.24 1.48749 19 −43.401 Bf 1.00000 (Values for the conditionalexpressions) (1) Σd/f = 6.095 (2) f2/f = 2.572 (3) d1/f = 1.380 (4) νRP− νRN = 41.48 (5) nRN − nRP = 0.299

EXAMPLE 3

A fisheye lens according to Example 3 of the first embodiment shown inFIG. 5 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a double convex shape with positive refractivepower, and a fourth lens component L4 being a cemented negative lenscomponent constructed by a positive lens element cemented with anegative lens element. The rear lens group G2 is composed of, in orderfrom the object, a fifth lens component L5 having a double convex shapewith positive refractive power, a sixth lens component L6 being acemented negative lens component constructed by a negative lens elementcemented with a positive lens element, and a seventh lens component L7having a double convex shape with positive refractive power.

Various values associating Example 3 are shown in Table 3.

TABLE 3 (Specification) f = 10.5 FNO = 2.8 2ω = 180.7 Bf = 38.0 TL =100.0 (Lens Data) Surface Number r d νd nd  1 84.102 2.50 46.63 1.81600 2 14.531 7.29 1.00000  3 166.427 2.00 49.61 1.77250  4 18.949 10.231.00000  5 31.103 4.13 25.43 1.80518  6 −43.681 2.68 1.00000  7 15.0552.99 52.42 1.51742  8 −31.790 1.00 42.72 1.83481  9 10.752 5.29 1.0000010 S 4.83 1.00000 11 ∞ 1.00 64.14 1.51633 12 ∞ 1.00 1.00000 13 34.8236.00 70.24 1.48749 14 −16.988 0.15 1.00000 15 −52.806 1.00 37.17 1.8340016 18.437 5.30 70.24 1.48749 17 −24.894 0.10 1.00000 18 33.005 4.5181.61 1.49700 19 −49.526 Bf 1.00000 (Values for the conditionalexpressions) (1) Σd/f = 5.905 (2) f2/f = 2.006 (3) d1/f = 1.155 (4) νRP− νRN = 36.86 (5) nRN − nRP = 0.343

EXAMPLE 4

A fisheye lens according to Example 4 of the first embodiment shown inFIG. 7 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a double convex shape with positive refractivepower, and a fourth lens component L4 being a cemented negative lenscomponent constructed by a positive lens element cemented with anegative lens element. The rear lens group G2 is composed of, in orderfrom the object, a fifth lens component L5 having a double convex shape,a sixth lens component L6 being a cemented negative lens componentconstructed by a negative lens element cemented with a positive lenselement, and a seventh lens component L7 having a double convex shapewith positive refractive power.

Various values associating Example 4 are shown in Table 4.

TABLE 4 (Specification) f = 10.5 FNO = 2.8 2ω = 180.7 Bf = 37.4 TL =99.4 (Lens Data) Surface Number r d νd nd  1 82.344 2.500 46.63 1.81600 2 14.141 7.295 1.00000  3 174.256 2.000 49.61 1.77250  4 20.126 10.2271.00000  5 31.775 4.130 25.43 1.80518  6 −45.533 2.679 1.00000  7 13.7212.994 52.42 1.51742  8 −43.798 1.000 42.72 1.83481  9 10.077 5.2931.00000 10 S 4.831 1.00000 11 ∞ 1.000 64.14 1.51633 12 ∞ 1.000 1.0000013 37.218 6.000 70.24 1.48749 14 −16.168 0.150 1.00000 15 −50.683 1.00037.17 1.83400 16 18.476 5.296 70.24 1.48749 17 −24.785 0.100 1.00000 1831.496 4.507 81.61 1.49700 19 −53.726 Bf 1.00000 (Values for theconditional expressions) (1) Σd/f = 5.905 (2) f2/f = 2.000 (3) d1/f =1.155 (4) νRP − νRN = 36.86 (5) nRN − nRP = 0.343

EXAMPLE 5

A fisheye lens according to Example 5 of the first embodiment shown inFIG. 9 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 being a cemented negative lens component constructedby a negative lens element cemented with a positive lens element, and afourth lens component L4 being a cemented positive lens componentconstructed by a positive lens element cemented with a negative lenselement. The rear lens group G2 is composed of, in order from theobject, a fifth lens component L5 having a double convex shape withpositive refractive power, a sixth lens component L6 being a cementednegative lens component constructed by a negative lens element cementedwith a positive lens element, and a seventh lens component L7 having adouble convex shape with positive refractive power.

Various values associating Example 5 are shown in Table 5.

TABLE 5 (Specification) f = 10.5 FNO = 2.8 2ω = 180.4 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 72.411 2.50 60.68 1.60311 2 15.735 15.92 1.00000  3 28.617 1.00 60.09 1.64000  4 12.674 4.851.00000  5 −23.498 1.35 60.09 1.64000  6 14.527 4.91 35.3 1.59270  7−25.302 0.50 1.00000  8 79.663 3.50 35.3 1.59270  9 −11.788 5.58 40.771.88300 10 −52.783 4.69 1.00000 11 S 3.75 1.00000 12 ∞ 1.00 64.141.51633 13 ∞ 1.41 1.00000 14 1042.319 2.33 70.24 1.48749 15 −25.279 1.001.00000 16 −134.056 1.00 25.43 1.80518 17 23.381 4.31 52.42 1.51742 18−30.241 0.10 1.00000 19 32.329 4.30 70.24 1.48749 20 −48.237 Bf 1.00000(Values for the conditional expressions) (1) Σd/f = 6.096 (2) f2/f =2.281 (3) d1/f = 1.033 (4) νRP − νRN = 38.87 (5) nRN − nRP = 0.308

EXAMPLE 6

A fisheye lens according to Example 6 of the first embodiment shown inFIG. 11 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 being a cemented negative lens component constructedby a negative lens element cemented with a positive lens element, and afourth lens component L4 being a cemented positive lens componentconstructed by a positive lens element cemented with a negative lenselement. The rear lens group G2 is composed of, in order from theobject, a fifth lens component L5 having positive refractive power and apositive meniscus shape with a stronger convex surface facing to animage, a sixth lens component L6 being a cemented positive lenscomponent constructed by a negative lens element cemented with apositive lens element, and a seventh lens component L7 having a doubleconvex shape.

Various values associating Example 6 are shown in Table 6.

TABLE 6 (Specification) f = 10.5 FNO = 2.8 2ω = 184.9 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 88.456 2.50 63.38 1.61800 2 15.969 17.68 1.00000  3 30.019 1.00 46.58 1.80400  4 14.663 7.681.00000  5 −19.270 1.00 63.38 1.61800  6 10.973 4.91 35.3 1.59270  7−21.519 0.50 1.00000  8 93.372 2.77 52.42 1.51742  9 −14.476 1.00 42.721.83481 10 −35.244 5.71 1.00000 11 S 4.36 1.00000 12 ∞ 1.00 64.141.51633 13 ∞ 2.29 1.00000 14 −82.227 1.94 81.61 1.49700 15 −28.439 1.001.00000 16 424.857 1.00 23.78 1.84666 17 29.343 3.90 81.61 1.49700 18−30.672 0.10 1.00000 19 39.192 3.68 70.24 1.48749 20 −52.057 Bf 1.00000(Values for the conditional expressions) (1) Σd/f = 6.095 (2) f2/f =2.477 (3) d1/f = 1.271 (4) νRP − νRN = 54.04 (5) nRN − nRP = 0.353

EXAMPLE 7

A fisheye lens according to Example 7 of the first embodiment shown inFIG. 13 cosists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 being a cemented negative lens component constructedby a negative lens element cemented with a positive lens element, afourth lens component L4 having a double convex shape with positiverefractive power, and a fifth lens component L5 being a cementednegative lens component constructed by a positive lens element cementedwith a negative lens element. The rear lens group G2 is composed of, inorder from the object, a sixth lens component L6 having positiverefractive power and a positive meniscus shape with a stronger convexsurface facing to an image, a seventh lens component L7 being a cementedpositive lens component constructed by a negative lens element cementedwith a positive lens element, and an eighth lens component L8 having adouble convex shape with positive refractive power.

Various values associating Example 7 are shown in Table 7.

TABLE 7 (Specification) f = 10.5 FNO = 2.8 2ω = 180.0 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 72.031 2.50 63.38 1.61800 2 15.316 9.37 1.00000  3 71.220 2.00 46.58 1.80400  4 21.254 13.561.00000  5 −19.566 1.00 63.38 1.61800  6 12.221 4.89 35.3 1.59270  7−29.281 0.10 1.00000  8 115.189 3.41 35.3 1.59270  9 −35.850 0.101.00000 10 38.990 2.97 54.68 1.51454 11 −16.347 1.00 42.72 1.83481 12128.223 5.85 1.00000 13 S 4.09 1.00000 14 ∞ 1.00 64.14 1.51633 15 ∞ 2.091.00000 16 −87.929 2.30 81.61 1.49700 17 −23.390 0.10 1.00000 18 252.6501.00 25.43 1.80518 19 27.543 3.68 81.61 1.49700 20 −36.636 0.10 1.0000021 33.519 3.36 81.61 1.49700 22 −95.905 Bf 1.00000 (Values for theconditional expressions) (1) Σd/f = 6.095 (2) f2/f = 2.369 (3) d1/f =1.241 (4) νRP − νRN = 56.18 (5) nRN − nRP = 0.308

EXAMPLE 8

A fisheye lens according to Example 8 of the first embodiment shown inFIG. 15 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a positive meniscus shape with a strongerconvex surface facing to an image with positive refractive power, afourth lens component L4 being a cemented positive lens componentconstructed by a negative lens element cemented with a positive lenselement, and a fifth lens component L5 being a cemented negative lenscomponent constructed by a positive lens element cemented with anegative lens element. The rear lens group G2 is composed of, in orderfrom the object, a sixth lens component L6 having positive refractivepower and a positive meniscus shape with a stronger convex surfacefacing to the image, a seventh lens component L7 being a cementedpositive lens component constructed by a negative lens element cementedwith a positive lens element, and an eighth lens component L8 having adouble convex shape with positive refractive power.

Various values associating Example 8 are shown in Table 8.

TABLE 8 (Specification) f = 10.5 FNO = 2.8 2ω = 180.5 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 85.910 2.50 63.38 1.61800 2 15.764 14.28 1.00000  3 64.782 2.00 46.58 1.80400  4 16.343 6.611.00000  5 −229.828 2.12 35.3 1.59270  6 −29.589 0.85 1.00000  7 −16.9811.50 63.38 1.61800  8 15.576 4.91 35.3 1.59270  9 −17.821 0.64 1.0000010 73.809 2.81 52.42 1.51742 11 −14.406 1.00 42.72 1.83481 12 −87.3165.74 1.00000 13 S 4.48 1.00000 14 ∞ 1.00 64.14 1.51633 15 ∞ 2.96 1.0000016 −84.470 2.17 81.61 1.49700 17 −24.565 0.10 1.00000 18 327.100 1.0023.78 1.84666 19 30.097 3.80 81.61 1.49700 20 −32.410 0.10 1.00000 2136.747 3.42 70.24 1.48749 22 −78.411 Bf 1.00000 (Values for theconditional expressions) (1) Σd/f = 6.090 (2) f2/f = 2.423 (3) d1/f =1.349 (4) νRP − νRN = 52.39 (5) nRN − nRP = 0.308

EXAMPLE 9

A fisheye lens according to Example 9 of the first embodiment shown inFIG. 17 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having positive refractive power and a positivemeniscus shape with a convex surface toward an image, a fourth lenscomponent L4 being a cemented negative lens component constructed by anegative lens element cemented with a positive lens element, and a fifthlens component L5 being a cemented negative lens component constructedby a positive lens element cemented with a negative lens element. Therear lens group G2 is composed of, in order from the object, a sixthlens component L6 being a cemented positive lens component constructedby a negative lens element cemented with a positive lens element, and aseventh lens component L7 having a double convex shape with positiverefractive power.

Various values associating Example 9 are shown in Table 9.

TABLE 9 (Specification) f = 10.56 FNO = 2.88 2ω = 180 Bf = 41.1 TL =103.7 (Lens Data) Surface Number r d νd nd  1 85.00 2.040 60.29 1.62041 2 16.45 8.320 1.00000  3 61.18 1.700 46.63 1.816  4 20.15 10.3161.00000  5 −204.00 3.430 43.69 1.72  6 −31.26 0.250 1.00000  7 −27.002.780 49.61 1.7725  8 11.48 4.930 28.46 1.72825  9 −55.84 5.865 1.0000010 59.67 2.570 52.42 1.51742 11 −14.20 1.410 42.72 1.83481 12 −76.003.660 1.00000 13 S 8.776 1.00000 14 −1732.00 1.490 23.78 1.84666 1529.90 4.020 81.61 1.497 16 −21.55 0.190 1.00000 17 44.74 3.210 58.541.6516 18 −44.74 Bf 1.00000 (Values for the conditional expressions) (1)Σd/f = 5.930 (2) f2/f = 2.320 (3) d1/f = 0.959 (4) νRP − νRN = 46.300(5) nRN − nRP = 0.270

EXAMPLE 10

A fisheye lens according to Example 10 of the first embodiment shown inFIG. 19 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a double concave shape, and a fourth lenscomponent L4 having a double convex shape, and a fifth lens component L5having a negative meniscus shape. The rear lens group G2 is composed of,in order from the object, a sixth lens component L6 having a positivemeniscus shape with a stronger convex surface facing to an image, aseventh lens component L7 being a cemented positive lens componentconstructed by a negative lens element cemented with a positive lenselement, and an eighth lens component L8 having a double convex shapewith positive refractive power.

Various values associating Example 10 are shown in Table 10.

TABLE 10 (Specification) f = 10.5 FNO = 2.88 2ω = 180 Bf = 39.0 TL =106.0 (Lens Data) Surface Number r d νd nd  1 91.145 2.5 55.52 1.69680 2 17.807 10.7158 1.00000  3 59.308 1.00 60.29 1.62041  4 23.682 17.16511.00000  5 −156.643 1.00 60.09 1.64000  6 15.400 1.50 1.00000  7 22.4863.9263 35.3 1.59270  8 −16.177 0.2 1.00000  9 −15.564 1 39.59 1.80440 10−27.069 7.7273 1.00000 11 S 4.6908 1.00000 12 ∞ 1.00 64.14 1.51633 13 ∞2.466 1.00000 14 −92.801 2.1463 48.87 1.53172 15 −34.586 1.00 1.00000 16−227.808 1.00 25.43 1.80518 17 27.770 4.0689 81.61 1.49700 18 −29.1460.1 1.00000 19 44.100 3.8181 70.24 1.48749 20 −34.128 Bf 1.00000 (Valuesfor the conditional expressions) (1) Σd/f = 6.383 (2) f2/f = 2.572 (3)d1/f = 1.513 (4) νRP − νRN = 41.477 (5) nRN − nRP = 0.299Second Embodiment

A fisheye lens according to a second embodiment of the present inventionis explained below.

A fisheye lens according to the second embodiment of the presentinvention consists of a front lens group having negative refractivepower and a rear lens group having positive refractive power locatingwith a space along the optical axis apart from the front lens group. Thefront lens group includes, in order from an object, a plurality ofnegative lens components, and two cemented lens components. At least oneof the two cemented lens components has a negative lens element to theobject side thereof and the negative lens element satisfies thefollowing conditional expression (6);1.0<|R|/f<4.0  (6)where f denotes the focal length of the fisheye lens and R denotes theradius of curvature of the object side surface of the negative lenselement arranged to the object side.

The front lens group has a negative refractive power component havingstrong divergent effect to deflect light ray entering from a wideincident range of 180 degrees to a direction parallel to the opticalaxis. By constructing the negative refractive power component of thefront lens group with a plurality of negative lens components,aberrations produced thereby can be spread to or shared by the pluralityof the negative lens components. Moreover, by arranging two cementedlens components, curvature of field, astigmatism and lateral chromaticaberration produced at the negative lens components can be corrected.

The first cemented lens component arranged to the object side iscomposed of, in order from the object, a negative lens element havinglow dispersion and a positive lens element having high dispersion andsatisfactory corrects lateral chromatic aberration. The second cementedlens component is composed of, in order from the object, a positive lenselement having low refractive index and a negative lens element havinghigh refractive index, satisfactorily corrects axial chromaticaberration, and suppresses production of curvature of field andastigmatism with securing sufficient back focal length.

In the second embodiment, the following conditional expressions (7)through (11) may be preferably further satisfied:4.0≦Σd/f≦10.0  (7)1.5≦f2/f≦4.0  (8)0.5<d1/f<2.0  (9)30≦vRP−vRN≦60  (10)0.2≦nRN−nRP≦0.45  (11)where f denotes the focal length of the fisheye lens, Σd denotes thedistance from the most object side lens surface to the most image sidelens surface of the fisheye lens when focusing at infinity, f2 denotesthe focal length of the rear lens group, d1 denotes the distance alongthe optical axis between the front lens group and the rear lens group,vRP denotes the mean value of Abbe numbers of the positive lens elementsin the lens components in the rear lens group, vRN denotes the meanvalue of Abbe numbers of the negative lens elements in the lenscomponents in the rear lens group, nRN denotes the mean value ofrefractive indices of the negative lens elements in the lens componentsin the rear lens group at d-line (λ=587.6 nm) and nRP denotes the meanvalue of refractive indices of the positive lens elements in the lenscomponents in the rear lens group at d-line (λ=587.6 nm).

In a fisheye lens according to the second embodiment of the presentinvention, conditional expression (6) is for controlling the size of theouter diameter of the negative lens component in the front lens groupwith securing sufficient peripheral quantity of light and back focallength. When the ratio |R|/f is higher than or equal to the upper limitor is less than or equal to the lower limit of conditional expression(6), the back focal length becomes too short to be able to be used for aSLR camera. Moreover, in order to pass light ray entering from a wideincident range of 180 degrees, the diameter (front lens diameter) of thelens locating to the most object side becomes extremely large, so thatthe whole lens becomes large in size and heavy in weight. In order toobtain good optical performance, it is preferable that the upper andlower limits are set to 3.5 and 1.5, respectively.

In a fisheye lens according to the second embodiment of the presentinvention, conditional expression (7) is for controlling the whole sizeand weight of the lens with securing sufficient back focal length andcorrecting off-axis aberrations. When the ratio Σd/f exceeds the upperlimit of conditional expression (7), the back focal length becomes tooshort to be able to be used for a SLR camera. Moreover, in order to passlight ray entering from a wide incident range of 180 degrees, thediameter (front lens diameter) of the lens locating to the most objectside becomes extremely large, so that the whole lens becomes large insize and heavy in weight. In order to obtain good optical performance,it is preferable that the upper limit is set to 8.0. On the other hand,when the ratio Σd/f falls below the lower limit of conditionalexpression (7), although the back focal length can be securedsufficiently, it becomes difficult to maintain the angle of view of 180degrees, so that it is undesirable. In order to obtain good opticalperformance, it is preferable that the lower limit is set to 5.0.

In a fisheye lens according to the second embodiment of the presentinvention, conditional expression (8) defines an appropriate range ofthe focal length of the rear lens group with securing sufficient backfocal length and correcting off-axis aberrations. When the ratio f2/fexceeds the upper limit of conditional expression (8), it becomesdifficult to secure the angle of view of 180 degrees as well as tocorrect astigmatism and coma, so that it is undesirable. On the otherhand, when the ratio f2/f falls below the lower limit of conditionalexpression (8), the back focal length becomes too short to be able to beused for a SLR camera.

In a fisheye lens according to the second embodiment of the presentinvention, conditional expression (9) defines an appropriate range ofthe distance along the optical axis between the front lens group and therear lens group. When the ratio d1/f is higher than or equal to theupper limit of conditional expression (9), lateral chromatic aberrationbecomes too large to correct properly. In addition, the back focallength becomes too short to be able to be used for a SLR camera. On theother hand, when the ratio d1/f is less than or equal to the lower limitof conditio al expression (9), it becomes difficult to secure the angleof view of 180 degrees as well as to correct astigmatism and coma, sothat it is undesirable.

In the above-described construction, production of lateral chromaticaberration unable to be fully suppressed by the front lens group can bemoderated by arranging a cemented lens component in the rear lens group.It is preferable to satisfy conditional expression (10).

In a fisheye lens according to the second embodiment of the presentinvention, conditional expression (10) defines an appropriate range ofdifference in Abbe numbers between the positive lens elements andnegative lens elements of the rear lens group in order to correctlateral chromatic aberration and axial chromatic aberration produced inthe front lens group. When the value vRP−vRN exceeds the upper limit ofconditional expression (10), it becomes difficult to construct the lenssystem by available existing glass materials as well as to correct axialchromatic aberration. On the other hand, when the value vRP−vRN fallsbelow the lower limit of conditional expression (10), lateral chromaticaberration produced by negative lens components in the front lens groupbecomes difficult to be corrected by the rear lens group and lateralchromatic aberration at g-line (λ=435.8 nm) tends to become negative, sothat it is undesirable.

In a fisheye lens according to the second embodiment of the presentinvention, conditional expression (11) is for correcting curvature offield and astigmatism. By setting refractive index of the negative lensas high as possible and that of the positive lens as low as possible,Petzval sum of the whole lens system can be lowered and curvature offield and astigmatism can be suppressed to be small. When the valuenRN−nRP exceeds the upper limit of conditional expression (11), thedifference in Abbe numbers between negative lens elements and positivelens elements has to be set larger, so that it becomes difficult tocorrect axial chromatic aberration produced in the front lens group. Onthe other hand, when the value nRN−nRP falls below the lower limit ofconditional expression (11), Petzval sum of the whole lens systembecomes large to produce large amount of curvature of field, so that itis undesirable.

When the focusing of the lens is carried out from infinity to closeobject, in order to prevent degradation of optical performance whenfocusing to a close object, it is preferable that the lens is moved tothe object side with increasing the distance between the front lensgroup and the rear lens group.

In the above-described construction, it is preferable that each lenssurface of the front and rear lens groups is constructed by a sphericalsurface or a plane surface without using an aspherical surface.Accordingly, it becomes easier to manufacture, assemble, and adjust eachlens element, so that manufacturing cost can be lowered.

Each example of the fisheye lens according to the second embodiment ofthe present invention is shown below.

FIGS. 9, 11, 13, and 15 are sectional views showing fisheye lensesaccording to Examples 11 through 14 of the second embodiment of thepresent invention, respectively. In respective sectional views, a planeparallel plate P, S and I denote a filter, an aperture stop, and animage plane respectively. The filter and the aperture stop are arrangedbetween the front lens group G1 and the rear lens group G2. The filterthat may be inserted into any place in the fisheye lens hassubstantially no effect on the lens. Moreover, if the fisheye lens doesnot have the filter, the basic optical performance of the lens is notsubstantially affected.

FIGS. 10, 12, 14, and 16 graphically show various aberrations of thefisheye lenses according to Examples 11 through 14 of the secondembodiment of the present invention, respectively. As is shown fromrespective graphs, each aberration is corrected preferably.

EXAMPLE 11

A fisheye lens according to Example 11 of the second embodiment shown inFIG. 9 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 being a cemented negative lens component constructedby a negative lens element cemented with a positive lens element, and afourth lens component L4 being a cemented positive lens componentconstructed by a positive lens element cemented with a negative lenselement. The rear lens group G2 is composed of, in order from theobject, a fifth lens component L5 having a double convex shape withpositive refractive power, a sixth lens component L6 being a cementednegative lens component constructed by a negative lens element cementedwith a positive lens element, and a seventh lens component L7 having adouble convex shape with positive refractive power.

Various values associating Example 11 are shown in Table 11.

TABLE 11 (Specification) f = 10.5 FNO = 2.8 2ω = 180.4 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 72.411 2.50 60.68 1.60311 2 15.735 15.92 1.00000  3 28.617 1.00 60.09 1.64000  4 12.674 4.851.00000  5 −23.498 1.35 60.09 1.64000  6 14.527 4.91 35.3 1.59270  7−25.302 0.50 1.00000  8 79.663 3.50 35.3 1.59270  9 −11.788 5.58 40.771.88300 10 −52.783 4.69 1.00000 11 S 3.75 1.00000 12 ∞ 1.00 64.141.51633 13 ∞ 1.41 1.00000 14 1042.319 2.33 70.24 1.48749 15 −25.279 1.001.00000 16 −134.056 1.00 25.43 1.80518 17 23.381 4.31 52.42 1.51742 18−30.241 0.10 1.00000 19 32.329 4.30 70.24 1.48749 20 −48.237 Bf 1.00000(Values for the conditional expressions)  (6) |R|/f = 2.238  (7) Σd/f =6.096  (8) f2/f = 2.281  (9) d1/f = 1.033 (10) νRP − νRN = 38.87 (11)nRN − nRP = 0.308

EXAMPLE 12

A fisheye lens according to Example 12 of the second embodiment shown inFIG. 11 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 being a cemented negative lens component constructedby a negative lens element cemented with a positive lens element, and afourth lens component L4 being a cemented positive lens componentconstructed by a positive lens element cemented with a negative lenselement. The rear lens group G2 is composed of, in order from theobject, a fifth lens component L5 having positive refractive power and apositive meniscus shape with a stronger convex surface facing to animage, a sixth lens component L6 being a cemented positive lenscomponent constructed by a negative lens element cemented with apositive lens element, and a seventh lens component L7 having a doubleconvex shape.

Various values associating Example 12 are shown in Table 12.

TABLE 12 (Specification) f = 10.5 FNO = 2.8 2ω = 184.9 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 88.456 2.50 63.38 1.61800 2 15.969 17.68 1.00000  3 30.019 1.00 46.58 1.80400  4 14.663 7.681.00000  5 −19.270 1.00 63.38 1.61800  6 10.973 4.91 35.3 1.59270  7−21.519 0.50 1.00000  8 93.372 2.77 52.42 1.51742  9 −14.476 1.00 42.721.83481 10 −35.244 5.71 1.00000 11 S 4.36 1.00000 12 ∞ 1.00 64.141.51633 13 ∞ 2.29 1.00000 14 −82.227 1.94 81.61 1.49700 15 −28.439 1.001.00000 16 424.857 1.00 23.78 1.84666 17 29.343 3.90 81.61 1.49700 18−30.672 0.10 1.00000 19 39.192 3.68 70.24 1.48749 20 −52.057 Bf 1.00000(Values for the conditional expressions)  (6) |R|/f = 1.835  (7) Σd/f =6.095  (8) f2/f = 2.477  (9) d1/f = 1.271 (10) νRP − νRN = 54.04 (11)nRN − nRP = 0.353

EXAMPLE 13

A fisheye lens according to Example 13 of the second embodiment shown inFIG. 13 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 being a cemented negative lens component constructedby a negative lens element cemented with a positive lens element, afourth lens component L4 having a double convex shape with positiverefractive power, and a fifth lens component L5 being a cementednegative lens component constructed by a positive lens element cementedwith a negative lens element. The rear lens group G2 is composed of, inorder from the object, a sixth lens component L6 having positiverefractive power and a positive meniscus shape with a stronger convexsurface facing to an image, a seventh lens component L7 being a cementedpositive lens component constructed by a negative lens element cementedwith a positive lens element, and an eighth lens component L8 having adouble convex shape with positive refractive power.

Various values associating Example 13 are shown in Table 13.

TABLE 13 (Specification) f = 10.5 FNO = 2.8 2ω = 180.0 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 72.031 2.50 63.38 1.61800 2 15.316 9.37 1.00000  3 71.220 2.00 46.58 1.80400  4 21.254 13.561.00000  5 −19.566 1.00 63.38 1.61800  6 12.221 4.89 35.3 1.59270  7−29.281 0.10 1.00000  8 115.189 3.41 35.3 1.59270  9 −35.850 0.101.00000 10 38.990 2.97 54.68 1.51454 11 −16.347 1.00 42.72 1.83481 12128.223 5.85 1.00000 13 S 4.09 1.00000 14 ∞ 1.00 64.14 1.51633 15 ∞ 2.091.00000 16 −87.929 2.30 81.61 1.49700 17 −23.390 0.10 1.00000 18 252.6501.00 25.43 1.80518 19 27.543 3.68 81.61 1.49700 20 −36.636 0.10 1.0000021 33.519 3.36 81.61 1.49700 22 −95.905 Bf 1.00000 (Values for theconditional expressions)  (6) |R|/f = 1.863  (7) Σd/f = 6.095  (8) f2/f= 2.369  (9) d1/f = 1.241 (10) νRP − νRN = 56.18 (11) nRN − nRP = 0.308

EXAMPLE 14

A fisheye lens according to Example 14 of the second embodiment shown inFIG. 15 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having positive refractive power and a positivemeniscus shape with a stronger convex surface facing to an image, afourth lens component L4 being a cemented positive lens componentconstructed by a negative lens element cemented with a positive lenselement, and a fifth lens component L5 being a cemented negative lenscomponent constructed by a positive lens element cemented with anegative lens element. The rear lens group G2 is composed of, in orderfrom the object, a sixth lens component L6 having positive refractivepower and a positive meniscus shape with a stronger convex surfacefacing to the image, a seventh lens component L7 being a cementedpositive lens component constructed by a negative lens element cementedwith a positive lens element, and an eighth lens component L8 having adouble convex shape with positive refractive power.

Various values associating Example 14 are shown in Table 14.

TABLE 14 (Specification) f = 10.5 FNO = 2.8 2ω = 180.5 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 85.910 2.50 63.38 1.61800 2 15.764 14.28 1.00000  3 64.782 2.00 46.58 1.80400  4 16.343 6.611.00000  5 −229.828 2.12 35.3 1.59270  6 −29.589 0.85 1.00000  7 −16.9811.50 63.38 1.61800  8 15.576 4.91 35.3 1.59270  9 −17.821 0.64 1.0000010 73.809 2.81 52.42 1.51742 11 −14.406 1.00 42.72 1.83481 12 −87.3165.74 1.00000 13 S 4.48 1.00000 14 ∞ 1.00 64.14 1.51633 15 ∞ 2.96 1.0000016 −84.470 2.17 81.61 1.49700 17 −24.565 0.10 1.00000 18 327.100 1.0023.78 1.84666 19 30.097 3.80 81.61 1.49700 20 −32.410 0.10 1.00000 2136.747 3.42 70.24 1.48749 22 −78.411 Bf 1.00000 (Values for theconditional expressions)  (6) |R|/f = 1.617  (7) Σd/f = 6.090  (8) f2/f= 2.423  (9) d1/f = 1.349 (10) νRP − νRN = 52.39 (11) nRN − nRP = 0.308Third Embodiment

A fisheye lens according to a third embodiment of the present inventionis explained below.

A fisheye lens according to the third embodiment of the presentinvention consists of a front lens group having negative refractivepower and a rear lens group having positive refractive power locatingwith a space along the optical axis apart from the front lens group. Therear lens group includes a cemented lens component and the followingconditional expressions (12) and (15) are satisfied. The rear lens groupmay additionally include the expression (13).Bf/f≧3.45  (12)0.5<d1/f<2.0  (15)4.0≦Σd/f≦10.0  (13)where f denotes the focal length of the fisheye lens, Bf denotes theback focal length, d1 denotes the distance along the optical axisbetween the front lens group and the rear lens group, and Σd denotes thedistance from the most object side lens surface to the most image sidelens surface of the fisheye lens when focusing at infinity.

The front lens group has a negative refractive power component havingstrong divergent effect to deflect light ray entering from a wideincident range of 180 degrees to a direction parallel to the opticalaxis. By constructing the negative refractive power component of thefront lens group with a plurality of negative lens componentsaberrations produced thereby can be spread to or shared by the pluralityof the negative refractive power components. In addition, by including acemented lens component in the front lens group, production of lateralchromatic aberration can be suppressed.

In the third embodiment, the following conditional expressions (14),(16) and (17) may be satisfied;1.5≦f2/f≦4.0  (14)30≦vRP−vRN≦60  (16)0.2≦nRN−nRP≦0.45  (17)where f2 denotes the focal length of the rear lens group, vRP denotesthe mean value of Abbe numbers of the positive lens components in therear lens group, vRN denotes the mean value of Abbe numbers of thenegative lens components in the rear lens group, nRN denotes the meanvalue of refractive indices of the negative lens components in the rearlens group at d-line (λ=587.6 nm) and nRP denotes the mean value ofrefractive indices of the positive lens components in the rear lensgroup at d-line (λ=587.6 nm).

The respective conditional expressions are explained blow.

Conditional expression (12) defines the relation between the focallength of the whole system and the back focal length of the fisheye lensaccording to the third embodiment of the present invention. When theratio Bf/f falls below the lower limit of conditional expression (12),it becomes difficult to use for a SLR camera, so that it is undesirable.

In a fisheye lens according to the third embodiment of the presentinvention, conditional expression (15) defines an appropriate range ofthe distance along the optical axis between the front lens group and therear lens group. When the ratio d1/f is higher than or equal to theupper limit of conditional expression (15), lateral chromatic aberrationbecomes too large to correct properly. In addition, the back focallength becomes too short to be able to be used for a SLR camera. On theother hand, when the ratio d1/f is less than or equal to the lower limitof conditional expression (15), it becomes difficult to secure the angleof view of 180 degrees as well as to correct astigmatism and coma, sothat it is undesirable. In order to obtain better optical performance,it is preferable to set the lower limit to 0.6.

In a fisheye lens according to the third embodiment of the presentinvention, conditional expression (13) is for controlling or determiningthe whole size and weight of the lens with securing sufficient backfocal length and correcting off-axis aberrations. When the ratio Σd/fexceeds the upper limit of conditional expression (13), the back focallength becomes too short to be able to be used for a SLR camera.Moreover, in order to pass light ray entering from a wide incident rangeof 180 degrees, the diameter (front lens diameter) of the lens componentlocating to the most object side becomes extremely large, so that thewhole lens becomes large in size and heavy in weight. In order to obtaingood optical performance, it is preferable that the upper limit is setto 8.0. On the other hand, when the ratio Σd/f falls below the lowerlimit of conditional expression (13), although the back focal length canbe secured sufficiently, it becomes difficult to maintain the angle ofview of 180 degrees, so that it is undesirable. In order to obtain goodoptical performance, it is preferable that the lower limit is set to5.0.

In a fisheye lens according to the third embodiment of the presentinvention, conditional expression (14) defines an appropriate range ofthe focal length of the rear lens group with securing sufficient backfocal length and correcting off-axis aberrations. When the ratio f2/fexceeds the upper limit of conditional expression (14), it becomesdifficult to secure the angle of view of 180 degrees as well as tocorrect astigmatism and coma, so that it is undesirable. On the otherhand, when the ratio f2/f falls below the lower limit of conditionalexpression (14), the back focal length becomes too short to be able tobe used for a SLR camera.

In the above-described construction, production of lateral chromaticaberration unable to be fully suppressed by the front lens group can bemoderated by arranging a cemented lens component in the rear lens group.It is preferable to satisfy conditional expression (16).

In a fisheye lens according to the third embodiment of the presentinvention, conditional expression (16) defines an appropriate range ofdifference in Abbe numbers between the positive lens element andnegative lens element in the rear lens group in order to correct lateralchromatic aberration and axial chromatic aberration produced in thefront lens group. When the value vRP−vRN exceeds the upper limit ofconditional expression (16), it becomes difficult to construct the lenssystem by available glass materials as well as to correct axialchromatic aberration. On the other hand, when the value vRP−vRN fallsbelow the lower limit of conditional expression (16), lateral chromaticaberration produced by negative lens components in the front lens groupbecomes difficult to be corrected by the rear lens group and lateralchromatic aberration at g-line (λ=435.8 nm) tends to become negative, sothat it is undesirable.

In a fisheye lens according to the third embodiment of the presentinvention, conditional expression (17) is for correcting curvature offield and astigmatism. By setting refractive index of the negative lenselement in the rear lens group G2 as high as possible and that of thepositive lens element as low as possible, Petzval sum of the fisheyelens can be lowered and curvature of field and astigmatism can besuppressed to be small. When the value nRN−nRP exceeds the upper limitof conditional expression (17), the difference in Abbe numbers betweennegative lens element and positive lens element has to be set larger, sothat it becomes difficult to correct axial chromatic aberration producedin the front lens group. On the other hand, when the value nRN−nRP fallsbelow the lower limit of conditional expression (17), Petzval sum of thewhole lens system becomes large to produce large amount of curvature offield, so that it is undesirable.

When the focusing of the lens is carried out from infinity to closeobject, in order to prevent degradation of optical performance whenfocusing to a close object, it is preferable that the lens is moved tothe object side with increasing the distance between the front lensgroup and the rear lens group.

In the above-described construction, it is preferable that each lenssurface of the front and rear lens groups is constructed by a sphericalsurface or a plane surface without using an aspherical surface.Accordingly, it becomes easier to manufacture, assemble, and adjust eachlens element, so that manufacturing cost can be lowered.

Each example of the fisheye lens according to the third embodiment ofthe present invention is shown below.

FIGS. 1, 3, 5, and 7 are sectional views showing fisheye lensesaccording to Examples 15 through 18 of the third embodiment of thepresent invention, respectively. In respective sectional views, a planeparallel plate P, S and I denote a filter, an aperture stop, and animage plane respectively. The filter and the aperture stop are arrangedbetween the front lens group G1 and the rear lens group G2. The filterthat may be inserted into any place in the fisheye lens hassubstantially no effect on the lens. Moreover, if the fisheye lens doesnot have the filter, the basic optical performance of the lens is notsubstantially affected.

FIGS. 2, 4, 6, and 8 graphically show various aberrations of the fisheyelenses according to Examples 15 through 18 of the third embodiment ofthe present invention, respectively. As is shown from respective graphs,each aberration is corrected preferably.

EXAMPLE 15

A fisheye lens according to Example 15 of the third embodiment shown inFIG. 1 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, and athird lens component L3 being a cemented positive lens componentconstructed by a negative lens element cemented with a positive lenselement. The rear lens group G2 is composed of, in order from theobject, a fourth lens component L4 having a double convex shape with astronger convex surface facing to an image, a fifth lens component L5being a cemented positive lens component constructed by a negative lenselement cemented with a positive lens element, and a sixth lenscomponent L6 having a double convex shape with positive refractivepower. All lens surfaces of the front and rear lens groups are composedof spherical surfaces.

Various values associating Example 15 are shown in Table 15.

TABLE 15 (Specification) f = 10.0 FNO = 2.8 2ω = 180.3 Bf = 38.0 TL =101.9 (Lens Data) Surface Number r d νd nd  1 70.000 1.50 60.68 1.60311 2 14.900 13.60 1.00000  3 36.500 1.00 49.61 1.77250  4 15.300 16.901.00000  5 −956.300 1.00 52.32 1.75500  6 10.250 3.60 31.07 1.68893  7−66.600 0.85 1.00000  8 S 8.20 1.00000  9 ∞ 1.00 64.14 1.51633 10 ∞ 3.981.00000 11 78.500 2.43 90.3 1.45600 12 −32.600 1.00 1.00000 13 −232.9001.00 23.78 1.84666 14 27.200 4.00 70.24 1.48749 15 −29.900 0.10 1.0000016 41.200 3.78 70.24 1.48749 17 −45.873 Bf 1.00000 (Values for theconditional expressions) (12) Bf/f = 3.80 (13) Σd/f = 6.39 (14) f2/f =2.53 (15) d1/f = 1.40 (16) νRP − νRN = 53.15 (17) nRN − nRP = 0.37

EXAMPLE 16

A fisheye lens according to Example 16 of the third embodiment shown inFIG. 3 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a double concave shape, and a fourth lenscomponent L4 being a cemented positive lens component constructed by apositive lens element cemented with a negative lens element. The rearlens group G2 is composed of, in order from the object, a fifth lenscomponent L5 having a positive meniscus shape with a stronger convexsurface facing to an image, a sixth lens component L6 being a cementednegative lens component constructed by a negative lens element cementedwith a positive lens element, and a seventh lens component L7 having adouble convex shape with positive refractive power. All lens surfaces ofthe front and rear lens groups are composed of spherical surfaces.

Various values associating Example 16 are shown in Table 16.

TABLE 16 (Specification) f = 10.5 FNO = 2.8 2ω = 180.6 Bf = 38.0 TL =102.0 (Lens Data) Surface Number r d νd nd  1 85.122 2.50 55.52 1.69680 2 16.625 9.29 1.00000  3 208.221 1.00 60.29 1.62041  4 28.271 17.171.00000  5 −284.146 1.00 60.09 1.64000  6 14.288 1.50 1.00000  7 18.6903.93 35.3 1.59270  8 −12.457 1.00 39.59 1.80440  9 −26.857 6.33 1.0000010 S 4.69 1.00000 11 ∞ 1.00 64.14 1.51633 12 ∞ 2.47 1.00000 13 −196.1932.15 48.87 1.53172 14 −27.112 1.00 1.00000 15 −179.223 1.00 25.431.80518 16 24.353 4.07 81.61 1.49700 17 −32.401 0.10 1.00000 18 40.9203.82 70.24 1.48749 19 −43.401 Bf 1.00000 (Values for the conditionalexpressions) (12) Bf/f = 3.62 (13) Σd/f = 6.10 (14) f2/f = 2.57 (15)d1/f = 1.38 (16) νRP − νRN = 41.48 (17) nRN − nRP = 0.30

EXAMPLE 17

A fisheye lens according to Example 17 of the third embodiment shown inFIG. 5 is composed of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a double convex shape with positive refractivepower, and a fourth lens component L4 being a cemented negative lenscomponent constructed by a positive lens element cemented with anegative lens element. The rear lens group G2 is composed of, in orderfrom the object, a fifth lens component L5 having a double convex shapewith positive refractive power, a sixth lens component L6 being acemented negative lens component constructed by a negative lens elementcemented with a positive lens element, and a seventh lens component L7having a double convex shape with positive refractive power. All lenssurfaces of the front and rear lens groups are composed of sphericalsurfaces.

Various values associating Example 17 are shown in Table 17.

TABLE 17 (Specification) f = 10.5 FNO = 2.8 2ω = 180.7 Bf = 38.0 TL =100.0 (Lens Data) Surface Number r d νd nd  1 84.102 2.50 46.63 1.81600 2 14.531 7.29 1.00000  3 166.427 2.00 49.61 1.77250  4 18.949 10.231.00000  5 31.103 4.13 25.43 1.80518  6 −43.681 2.68 1.00000  7 15.0552.99 52.42 1.51742  8 −31.790 1.00 42.72 1.83481  9 10.752 5.29 1.0000010 S 4.83 1.00000 11 ∞ 1.00 64.14 1.51633 12 ∞ 1.00 1.00000 13 34.8236.00 70.24 1.48749 14 −16.988 0.15 1.00000 15 −52.806 1.00 37.17 1.8340016 18.437 5.30 70.24 1.48749 17 −24.894 0.10 1.00000 18 33.005 4.5181.61 1.49700 19 −49.526 Bf 1.00000 (Values for the conditionalexpressions) (12) Bf/f = 3.62 (13) Σd/f = 5.90 (14) f2/f = 2.01 (15)d1/f = 1.15 (16) νRP − νRN = 36.86 (17) nRN − nRP = 0.34

EXAMPLE 18

A fisheye lens according to Example 18 of the third embodiment shown inFIG. 7 consists of a front lens group G1 having negative refractivepower, and a rear lens group G2 having positive refractive powerarranged with a space along the optical axis apart from the front lensgroup G1. The front lens group G1 is composed of, in order from anobject, a first lens component L1 having a negative meniscus shape witha convex surface toward the object, a second lens component L2 having anegative meniscus shape with a convex surface toward the object, a thirdlens component L3 having a double convex shape with positive refractivepower, and a fourth lens component L4 being a cemented negative lenscomponent constructed by a positive lens element cemented with anegative lens element. The rear lens group G2 is composed of, in orderfrom the object, a fifth lens component L5 having a double convex shape,a sixth lens component L6 being a cemented negative lens componentconstructed by a negative lens element cemented with a positive lenselement, and a seventh lens component L7 having a double convex shapewith positive refractive power. All lens surfaces of the front and rearlens groups are composed of spherical surfaces. In each exampledescribed above, it is needless to say that any surface may be a planesurface.

Various values associating Example 18 are shown in Table 18.

TABLE 18 (Specification) f = 10.5 FNO = 2.8 2ω = 180.7 Bf = 37.4 TL =99.4 (Lens Data) Surface Number r d νd nd  1 82.344 2.500 46.63 1.81600 2 14.141 7.295 1.00000  3 174.256 2.000 49.61 1.77250  4 20.126 10.2271.00000  5 31.775 4.130 25.43 1.80518  6 −45.533 2.679 1.00000  7 13.7212.994 52.42 1.51742  8 −43.798 1.000 42.72 1.83481  9 10.077 5.2931.00000 10 S 4.831 1.00000 11 ∞ 1.000 64.14 1.51633 12 ∞ 1.000 1.0000013 37.218 6.000 70.24 1.48749 14 −16.168 0.150 1.00000 15 −50.683 1.00037.17 1.83400 16 18.476 5.296 70.24 1.48749 17 −24.785 0.100 1.00000 1831.496 4.507 81.61 1.49700 19 −53.726 Bf 1.00000 (Values for theconditional expressions) (12) Bf/f = 3.62 (13) Σd/f = 5.90 (14) f2/f =2.00 (15) d1/f = 1.15 (16) νRP − νRN = 36.86 (17) nRN − nRP = 0.34

As described above, the present invention makes it possible to provide afisheye lens, in spite of its compactness, having an angle of view of180 degrees and a fast f-number of 2.8 with securing sufficient backfocal length and suitable for a digital camera.

Additional advantages and modification will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A fisheye lens comprising: a front lens group having negativerefractive power, and a rear lens group having positive refractive powerlocating with a space along the optical axis apart from the front lensgroup, the front lens group comprising a plurality of negative lenscomponents; the rear lens group comprising at least one cemented lenscomponent; and the following conditional expression is satisfied:4.0≦Σd/f≦10.0 where f denotes the focal length of the fisheye lens andΣd denotes the distance from the most object side lens surface to themost image side lens surface of the fisheye lens when focusing atinfinity.
 2. The fisheye lens according to claim 1, wherein the frontlens group includes at least one cemented lens component locating to theimage side of the plurality of negative lens components.
 3. The fisheyelens according to claim 2, wherein the following conditional expressionis satisfied:1.5≦f2/f≦4.0 where f2 denotes the focal length of the rear lens group.4. The fisheye lens according to claim 3, wherein the rear lens groupincludes, in order from the object, a positive lens component, acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a positive lens component.
 5. Thefisheye lens according to claim 4, wherein the following conditionalexpression is satisfied:30≦vRP−vRN≦60 where vRP denotes the mean value of Abbe numbers of thepositive lens elements in the lens components in the rear lens group andvRN denotes the mean value of Abbe numbers of the negative lens elementsin the lens components in the rear lens group.
 6. The fisheye lensaccording to claim 4, wherein the following conditional expression issatisfied;0.2≦nRN−nRP≦0.45 where nRN denotes the mean value of refractive indicesof the negative lens elements in the lens components in the rear lensgroup at d-line (λ=587.7 nm) and nRP denotes the mean value ofrefractive indices of the positive lens elements in the lens componentsin the rear lens group at d-line (λ=587.6 nm).
 7. The fisheye lensaccording to claim 1, wherein the following conditional expression issatisfied:1.5≦f2/f≦4.0 where f2 denotes the focal length of the rear lens group.8. The fisheye lens according to claim 1, wherein the rear lens groupincludes, in order from the object, a positive lens component, acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a positive lens component.
 9. Thefisheye lens according to claim 1, wherein the following conditionalexpression is satisfied:0.5≦d1/f≦2.0 where d1 denotes the distance along the optical axisbetween the front lens group and the rear lens group.
 10. The fisheyelens according to claim 1, wherein each of all lens surfaces of thefront lens group and the rear lens group is constructed by any one of aspherical surface and a plane surface.
 11. The fisheye lens according toclaim 1, wherein when the focusing of the lens is moved from infinity toclose object, the lens is moved to the object side with increasing thedistance between the front lens group and the rear lens group.
 12. Afisheye lens comprising: a front lens group having negative refractivepower, and a rear lens group having positive refractive power locatingwith a space along the optical axis apart from the front lens group; thefront lens group comprising, in order from an object, a plurality ofnegative lens components, and two cemented lenses components; at leastone of the two cemented lens components including a negative lenselement arranged to the object side, which the negative lens elementsatisfies the following conditional expression:1.0<|R|/f<4.0 where f denotes the focal length of the fisheye lens and Rdenotes the radius of curvature of the object side surface of saidnegative lens element arranged to the object side.
 13. The fisheye lensaccording to claim 12, wherein the front lens group includes, in orderfrom the object, a first lens component having a negative meniscus shapewith a convex surface toward the object; a second lens component havinga negative meniscus shape with a convex surface toward the object; athird lens component including a cemented lens component constructed bya negative lens element cemented with a positive lens element; and afourth lens component including a cemented lens component constructed bya positive lens element cemented with a negative lens element; whereinsaid plurality of negative lens components are composed of the firstlens component and the second lens component, and the two cemented lenscomponents are composed of a third lens component and a fourth lenscomponent.
 14. The fisheye lens according to claim 12, wherein the frontlens group includes, in order from the object, a first lens componenthaving a negative meniscus shape with a convex surface toward theobject; a second lens component having a negative meniscus shape with aconvex surface toward the object; a third lens component including acemented lens component constructed by a negative lens element cementedwith a positive lens element; a fourth lens component including apositive lens element; and a fifth lens component including a cementedcomponent lens constructed by a positive lens element cemented with anegative lens element; wherein said plurality of negative lenscomponents are composed of said first lens component and said secondlens component, and said two cemented lens components are composed ofsaid third lens component and said fifth lens component.
 15. The fisheyelens according to claim 12, wherein the front lens group includes, inorder from the object, a first lens component having a negative meniscusshape with a convex surface toward the object; a second lens componenthaving a negative meniscus shape with a convex surface toward theobject; a third lens component including a positive lens element; afourth lens component including a cemented component lens constructed bya negative lens element cemented with a positive lens element; and afifth lens component including a cemented lens component constructed bya positive lens element cemented with a negative lens element; whereinsaid plurality of negative lens components are composed of said firstlens component and said second lens component, and said two cementedlenses are composed of said fourth lens component and said fifth lenscomponent.
 16. The fisheye lens according to claim 12, wherein thefollowing conditional expression is satisfied:4.0≦Σd/f≦10.0 where f denotes the focal length of the fisheye lens andΣd denotes the distance from the most object side lens surface to themost image side lens surface of the fisheye lens when focusing atinfinity.
 17. The fisheye lens according to claim 16, wherein thefollowing conditional expression is satisfied:1.5≦f2/f≦4.0 where f2 denotes the focal length of the rear lens group.18. The fisheye lens according to claim 12, wherein the rear lens groupincludes, in order from the object, a positive lens component, acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a positive lens component.
 19. Thefisheye lens according to claim 18, wherein the following conditionalexpression is satisfied:30≦vRP−vRN≦60 where vRP denotes the mean value of Abbe numbers of thepositive lens elements in the lens components in the rear lens group andvRN denotes the mean value of Abbe numbers of the negative lens elementsin the lens components in the rear lens group.
 20. The fisheye lensaccording to claim 18, wherein the following conditional expression issatisfied:0.2≦nRK–nRP≦0.45 where nRN denotes the mean value of refractive indicesof the negative lens elements in the lens components in the rear lensgroup at d-line (λ=587.6 nm) and nRP denotes the mean value ofrefractive indices of the positive lens elements in the lens componentsin the rear lens group at d-line (λ=587.6 nm).
 21. The fisheye lensaccording to claim 12, wherein the following conditional expression issatisfied:0.5<d1/f<2.0 where d1 denotes the distance along the optical axisbetween the front lens group and the rear lens group.
 22. The fisheyelens according to claim 12, wherein each of all lens surfaces of thefront lens group and the rear lens group is constructed by any one of aspherical surface and a plane surface.
 23. The fisheye lens according toclaim 12, wherein when the focusing of the lens is moved from infinityto close object, the lens is moved to the object side with increasingthe distance between the front lens group and the rear lens group.
 24. Afisheye lens comprising: a front lens group having negative refractivepower, and a rear lens group having positive refractive power locatingwith a space along the optical axis apart from the front lens group, thefront lens group comprising, in order from an object, a plurality ofnegative lens components, and a cemented lens component; and the rearlens group comprising a cemented lens component; wherein the followingconditional expressions are satisfied:Bf/f≧3.450.5<d1/f<2.0 where Bf denotes the back focal length of the fisheye lens,d1 denotes the distance along the optical axis between the most imageside lens surface of the front lens group and the most object side lenssurface of the rear lens group, and f denotes the focal length of thefisheye lens.
 25. The fisheye lens according to claim 24, wherein thefollowing conditional expression is satisfied:4.0≦Σd/f≦10.0 where f denotes the focal length of the fisheye lens andΣd denotes the distance from the most object side lens surface to themost image side lens surface of the fisheye lens when focusing atinfinity.
 26. The fisheye lens according to claim 24, wherein the frontlens group includes, in order from the object, a first lens componenthaving a negative meniscus shape with a convex surface toward theobject, a second lens component having a negative meniscus shape with aconvex surface toward the object, and a third lens component including acemented lens component constructed by a negative lens element cementedwith a positive lens element.
 27. The fisheye lens according to claim24, wherein the front lens group includes, in order from the object, afirst lens component having a negative meniscus shape with a convexsurface toward the object, a second lens component having a negativemeniscus shape with a convex surface toward the object, a third lenscomponent including only a negative lens element, and a fourth lenscomponent including a cemented lens constructed by a positive lenselement cemented with a negative lens element.
 28. The fisheye lensaccording to claim 24, wherein the front lens group includes, in orderfrom the object, a first lens component having a negative meniscus shapewith a convex surface toward the object, a second lens component havinga negative meniscus shape with a convex surface toward the object; athird lens component including only a positive lens element, and afourth lens component including a cemented lens component constructed bya positive lens element cemented with a negative lens element.
 29. Thefisheye lens according to claim 24, wherein the rear lens groupincludes, in order from the object, a positive lens component, acemented lens component constructed by a negative lens element cementedwith a positive lens element, and a positive lens component.
 30. Thefisheye lens according to claim 29, wherein the following conditionalexpression is satisfied:1.5≦f2/f≦4.0 where f2 denotes the focal length of the rear lens group.31. The fisheye lens according to claim 29, wherein component thefollowing conditional expression is satisfied;30≦vRP−vRN≦60 where vRP denotes the mean value of Abbe numbers of thepositive lens elements in the lens components in the rear lens group andvRN denotes the mean value of Abbe numbers of the negative lens elementsin the lens components in the rear lens group.
 32. The fisheye lensaccording to claim 29, wherein the following conditional expression issatisfied;0.2≦nRN−nRP≦0.45 where nRN denotes the mean value of refractive indicesof the negative lens elements in the lens components in the rear lensgroup at d-line (λ=587.6 nm) and nRP denotes the mean value ofrefractive indices of the positive lens elements in the lens componentsin the rear lens group at d-line (λ=587.6 nm).
 33. The fisheye lensaccording to claim 24, wherein each of all lens surfaces of the frontlens group and the rear lens group is constructed by any one of aspherical surface and a plane surface.
 34. The fisheye lens according toclaim 24, wherein when the focusing of the lens is moved from infinityto close object, the lens is moved to the object side with increasingthe distance between the front lens group and the rear lens group.
 35. Amethod for forming an image of an object, comprising: providing afisheye lens including a front lens group having negative refractivepower and a rear lens group having positive refractive power, the frontlens group having a plurality of negative lens components, the rear lensgroup being located apart from the front lens group with a space alongan optical axis and having at least one cemented lens component, thefollowing conditional expression being satisfied:4.0≦Σd/f≦10.0 where f denotes the focal length of the fisheye lens andΣd denotes the distance from the most object side lens surface to themost image side lens surface of the fisheye lens when focusing oninfinity; and using the fisheye lens to form the image of the object.36. The method according to claim 35, wherein the front lens groupincludes at least one cemented lens component located to the image sideof the plurality of negative lens components.
 37. A method for formingan image of an object, comprising: providing a fisheye lens including afront lens group having negative refractive power and a rear lens grouphaving positive refractive power, the front lens group having, in orderfrom the object, a plurality of negative lens components and twocemented lens components, at least one of the two cemented lenscomponents including a negative lens element arranged to the objectside, the rear lens group being located apart from the front lens groupwith a space along an optical axis, the following conditional expressionbeing satisfied by the negative lens element:1.0<|R|/f<4.0 where f denotes the focal length of the fisheye lens and Rdenotes the radius of curvature of the object side surface of thenegative lens element; and using the fisheye lens to form the image ofthe object.
 38. A method for forming an image of an object, comprising:providing a fisheye lens including a front lens group having negativerefractive power and a rear lens group having positive refractive power,the front lens group having, in order from an object, a plurality ofnegative lens components and a cemented lens component, the rear lensgroup being located apart from the front lens group with a space alongan optical axis and having a cemented lens component; the followingconditional expressions being satisfied:Bf/f≧3.450.5<d1/f<2.0 where Bf denotes the back focal length of the fisheye lens,d1 denotes the distance along the optical axis between the most imageside lens surface of the front lens group and the most object side lenssurface of the rear lens group, and f denotes the focal length of thefisheye lens; and using the fisheye lens to form the image of theobject.